Combining Anisotropic Etching and PDMS Casting for Three-Dimensional Analysis of Laser Ablation ProcessesValentine Grimaudo,*,† Pavel Moreno-García,† Alena Cedeno Lopez,† Andreas Riedo,‡

Reto Wiesendanger,‡ Marek Tulej,‡ Cynthia Gruber,§ Emanuel Lortscher,§ Peter Wurz,‡

and Peter Broekmann*,†

†Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland‡Physics Institute, Space Research and Planetary Sciences, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland§IBM Research - Zurich, Science and Technology Department, Saumerstrasse 4, CH-8803 Ruschlikon, Switzerland

*S Supporting Information

ABSTRACT: State-of-the-art laser ablation (LA) depth-profiling techni-ques (e.g. LA-ICP-MS, LIBS, and LIMS) allow for chemical compositionanalysis of solid materials with high spatial resolution at micro- andnanometer levels. Accurate determination of LA-volume is essential tocorrelate the recorded chemical information to the specific location insidethe sample. In this contribution, we demonstrate two novel approachestowards a better quantitative analysis of LA craters with dimensions atmicrometer level formed by femtosecond-LA processes on single-crystalline Si(100) and polycrystalline Cu model substrates. For ourparametric crater evolution studies, both the number of applied lasershots and the pulse energy were systematically varied, thus yielding 2Dmatrices of LA craters which vary in depth, diameter, and crater volume. To access the 3D structure of LA craters formed onSi(100), we applied a combination of standard lithographic and deep reactive-ion etching (DRIE) techniques followed by a HR-SEM inspection of the previously formed crater cross sections. As DRIE is not applicable for other material classes such as metals,an alternative and more versatile preparation technique was developed and applied to the LA craters formed on the Cu substrate.After the initial LA treatment, the Cu surface was subjected to a polydimethylsiloxane (PDMS) casting process yielding a moldbeing a full 3D replica of the LA craters, which was then analyzed by HR-SEM. Both approaches revealed cone-like shapedcraters with depths ranging between 1 and 70 μm and showed a larger ablation depth of Cu that exceed the one of Si by a factorof about 3.

Laser ablation (LA) techniques utilizing ultrashort pulsesare increasingly attracting attention in fundamental

research, industry, and medicine.1,2 The fact that any kind ofsolid (electrically conductive or dielectric) can be ablatedwithin seconds without extensive and time-consuming samplepreparation makes these LA-based techniques highly interestingfor chemical analyses that are particularly focusing on small andlocally confined sample spots (⌀ at μm and sub-μm regime).These analytical techniques are applicable in various researchfields ranging from the analysis of solids in geology3,4 to theinvestigation of multicomponent structures in photovoltaic5

and semiconductor industry.6 LA inductively coupled plasmamass spectrometry (LA-ICP-MS),5,7−10 laser ionization massspectrometry (LIMS),4,6,11−16 and laser-induced breakdownspectroscopy (LIBS)9,17−19 are, among others, analyticaltechniques providing quantitative information on the chemicalcomposition of solids down to trace abundances with highspatial resolution at micrometer and nanometer levels. Witheach laser shot, a certain volume of sample material is ablatedand subsequently analyzed in the gas-phase. The successiveapplication of laser shots on a single spot of the sample allows

for the chemical analysis of the ablated volume and its chemicalreconstruction in depth. Further extending this depth profilingapproach to a 2D raster mode provides means of a full chemicalanalysis of the sample in three dimensions.9,10,14,17 However,the reliability of such LA-based chemical analyses crucially relieson the accurate determination of the spatial structure, e.g. thevolume, of the sample material removed during LA events. Thisis actually the basis for a reliable correlation of chemicalinformation (derived e.g. from the MS analysis) to specificlocations inside the sample or crater and becomes particularlyrelevant when dealing with cavities having large (depth-to-diameter) aspect ratios. It has already been shown, however,that the spatial structure of the ablation volume can significantlydeviate from an ideal cylindrical geometry.20,21 Instead, moretapered, cone-like cross sections were observed (denoted as“craters”). Such a deviation from an ideal ablation process hasconsiderable implications on the reconstruction of the chemical

Received: November 2, 2017Accepted: January 20, 2018Published: February 5, 2018

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pubs.acs.org/acCite This: Anal. Chem. 2018, 90, 2692−2700

© 2018 American Chemical Society 2692 DOI: 10.1021/acs.analchem.7b04539Anal. Chem. 2018, 90, 2692−2700

sample composition.22 Additionally, element fractionationchanges significantly as the crater depth evolves sincecondensation rate of ablated materials onto the side-wallscrucially depends on the crater depth. These effects have beenattributed to an ablation mechanism which changes at anadvanced depth stage from photothermal- to plasma-dominatedLA. Interactions between the plasma formed during the LA andthe inner walls of the craters need to be taken into account inparticular for aspect ratios ≥5−6.20,21 Further consequences ofthe irregularly shaped ablation craters are, among others, (i) anonuniform irradiance on the illuminated surface,20 (ii) anundesired self-focusing of the beam to the bottom of theablated zone,23 and (iii) the change of the ablation rate20 as theeffective illuminated area changes with progressing ablation.Understanding these effects is essential for a truly quantitativethree-dimensional chemical analysis of solid materials at smallspatial scales. This would further allow for parametric studieswhich are intended to achieve a clear correlation of specificablation parameters (wavelength, incident angle, pulse duration,repetition rate, fluence, number of applied laser pulses,etc.)1,24−26 to the resulting ablation volume and spatialstructure of the LA craters.To date, several methods have been employed to analyze LA

craters including atomic force microscopy (AFM),27,28 scanningelectron microscopy (SEM),29−31 white-light interferome-try,20,32−34 confocal microscopy,35 and optical microscopy oftransparent materials.20−22 A more versatile approach is basedon focused-ion beam (FIB) milling followed by SEM imagingof the cross-sectioned LA craters.36,37 A severe drawback of thistechnique, however, is related to the fact that only a singlecrater can be subjected to the ion milling at the same timewhich makes the FIB approach rather time-consuming andcostly in particular for more extended parametric studies on theLA crater evolution. Furthermore, ion implantation and ionchanneling effects might alter the crater characteristics uponmilling, thus leading to a falsified analysis of crater crosssections.In this contribution, we report two novel strategies which

allow a fast and reliable preparation of complete batches of 2DLA crater matrices (pulse energies, number of applied pulses)as basis for both a comprehensive qualitative and quantitativeanalysis of the LA characteristics and crater evolution.

Our first approach has its origin in industrial semiconductorprocessing and employs a combination of standard lithographicpatterning techniques and anisotropic selective Si etching(“Bosch-process”). Applied to 2D matrices of LA craters, thisapproach results in well-defined rows of partially cleared LAcraters that thus become accessible for cross-sectional SEMinspection.The second approach is based on a polydimethylsiloxane

(PDMS) casting process that yields a full three-dimensionalmold of the previously formed crater with all its features.Instead of analyzing the original LA craters in very constraintgeometries, their negative replica is analyzed by SEM. Thiscasting technique is nondestructive, applicable to any kind ofsolid substrate and is employable also at intermediate LA steps.It is therefore considered much more versatile than thecombined lithography and Si etching approach, which remainsrestricted to those (semiconductor) materials (e.g., Si) that canbe etched anisotropically.In our present study, 2D matrices of crater features were

produced on Si(100) and Cu substrates by means of afemtosecond (fs) laser ablation process. The HR-SEM analysisof either the crater cross sections or the crater replica wascomplemented by top-down optical white-light and atomicforce microscopic (AFM) inspection of the crater openingsprior to the etching or casting treatments.Both approaches discussed herein allow for a detailed

quantitative description of the crater evolution process.We expect that in particular the PDMS casting method will

significantly contribute in future to a more accurate spatialanalysis of trace elements in LA-based depth profilingexperiments.

■ MATERIALS AND METHODSSamples. LA craters were produced on single crystalline

Si(100) and polycrystalline Cu model samples whichsignificantly differ in their physical characteristics (e.g., thermalconductivity). The Si wafer coupons were provided by BASFTaiwan Ltd. The Cu-foil specimen was purchased from AlfaAesar (99.9% purity, 0.254 mm thickness).

Laser Ablation. As further improvement to our previousmass spectrometric measurements using a miniature LIMSsystem6,12,27 a beam shaper (focal-π shaper, AdlOptica GmbH,Germany) was introduced in the beam delivery system

Figure 1. (a) Schematic representation of the lithographic etching approach applied to investigate the morphological characteristics of the laser-induced craters on the Si sample. Only two “Bosch” cycles are shown in the third panel of the figure. (b) Schematic representation of the PDMS-based approach applied to investigate the morphological characteristics of the laser generated craters on the Cu sample.

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converting the Gaussian profile of the laser beam from the fs-laser system (τ ∼ 190 fs, λ = 775 nm) into a quasi-flat-topprofile at focal plane. The change in the group velocitydispersion is negligible as the material used for the optical partsin the shaper is fused silica.The basis for our comprehensive study on the laser-matter

interaction and the LA crater evolution was the systematicvariation of (i) the number of applied laser shots and (ii) thelaser pulse energy at multiple surface locations. For each pulseenergy (varied from 0.9 to 3.3 μJ), craters were generated for aramp of laser shots (ramped from 100 to 10 000 shots), thusleading to well-defined matrices of LA craters differing in theirlateral dimension, depth, and volume.All craters were analyzed by AFM and high-resolution-SEM

(HR-SEM). Further technical details about these measure-ments are reported in the Supporting Information (SI).Selective Anisotropic Etching of Silicon. To determine

the morphological characteristics of fs-induced LA craters onthe Si wafer, we used standard lithographic and deep reactive-ion etching techniques (DRIE). Parallel rectangular rows werelithographically patterned on the Si sample in such a way thathalf of the craters across their center were covered by thepolymer mask (see Figure 1a, masking step, and SI Figure S5).The pitch between adjacent rectangles was identical to the pitchbetween craters to ensure that the subsequent etching wouldprovide optimal matching to the craters’ centers. A “Bosch-process” of alternating etching-passivation cycles was thenapplied to achieve ∼80 μm vertical etching of the unmasked Sisurface (see third panel in Figure 1a; Alcatel 1800 W, etch rateof 5 μm/min). The resulting cross-sectioned LA craters werethen analyzed by tilted SEM imaging (Hitachi S-3000NScanning Electron Microscope (Japan), accelerating voltage of25 keV at a working distance of 10 mm) after resist removal inacetone and cleaning in isopropanol. The results aresummarized in Figure 4a, and a detailed image of a samplecrater is displayed in Figure 4b (3000 shots at 1.4 μJ).Negative Micro-Molding of Copper Craters. Because

the selective anisotropic etching discussed in the previoussection is suitable only for Si substrates, we employed analternative polydimethylsiloxane (PDMS)-based approach toobtain a three-dimensional replica of the LA craters on the Cusample (see Figure 1b). This approach allows the fabrication offlexible structures in the micrometer and sub-micrometer range

for microcontact printing and three-dimensional replicamolding.38

In a first step, a thin (octafluorocyclobutane-based)antiadhesion coating (∼50 nm) was deposited on the Cusample to avoid undesirable sticking between the PDMS andthe Cu substrate after baking. A custom-made mixture ofpolydimethylsiloxane (PDMS) derivative (Sylgard 184 fromSigma-Aldrich), was then poured over the Cu surface inside aTeflon-coated Aluminum compartment, filling the ablatedcavities. To cure the resin, the stack was placed inside avacuum furnace where the temperature was set to 60 °C for aduration of 12 h. This way any enclosed gas inside the PDMSmaterial or captured below the PDMS in enclosed volumes, e.g.at the craters’ bottom, is released by diffusion through thePDMS and a conformal replica is casted. Finally, the negativePDMS mold was gently released from the Cu sample providinga nanometer-accurate template of the ablated cavities of the Cusubstrate. This way, even the back-bending at the bottom of thecraters was reproduced. The high elasticity of PDMS moldavoids inelastic deformation of the Cu replica during thestripping.38 To study the characteristic dimensions of thePDMS negative by tilted SEM imaging, the mold was coated bya thin 5 nm Au/Pd conductive metal layer preventing anycharge accumulations on the surface by the released electrons(see panels c and d in Figure 4, Zeiss Leo SEM, 3 kVacceleration voltage, 30 μm aperture).

■ RESULTS AND DISCUSSIONTop-Down Crater Analysis. The LA characteristics for Si

and Cu target materials were comprehensively investigated bymeans of top-down SEM inspection and complementary AFManalysis. Figure S-1 shows optical microscope images with asurvey of crater arrays on the Cu and the Si sample,respectively, generated by a systematic variation of the laserpulse energy (0.9−3.3 μJ) and the total number of laser pulses(100−10 000). Figure 2 displays representative top-down SEMimages of Cu (panels a and c) and Si (panels f and h) samplesafter applying 100 and 10 000 laser shots at pulse energies of∼2.0 μJ. From the SEM inspection, it becomes obvious that thetotal surface area affected by the laser beam is much moreextended for Si than for Cu. However, the deeper central partsof these zones have in both cases similar lateral dimensions. Weconsider those areas affected by the laser beam as “craters”where massive ablation of substrate material takes place upon

Figure 2. HR-SEM images of LA craters illustrating the distinct laser-matter interaction of Cu and Si for a pulse energy of ∼2.0 μJ and differentnumber of applied laser shots. (a) and (f) Cu and Si crater generated with 100 consecutive laser shots each. (b) and (g) central part of Cu and Sicrater, respectively, indicated by the red frame in (a) and (f). (c) and (h) Cu and Si crater formed by 10 000 laser shots each. (d) and (i) show thecentral part, (e) and (j) the border part of the Cu and Si crater in (c) and (h) (indicated by the green and blue frame), respectively.

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laser irradiation, whereas significant changes in the surfacemorphology might occur in the outer parts of the zones affectedby the laser light even without any net removal of samplematerial. Irradiation of the Cu and the Si surfaces by the fs-laserbeam leads in both cases to crater features having an ellipticalappearance (Figure 2c,h). We attribute the observed ellipticalcrater shape to the particular polarization characteristics of theused laser light (linear)39 and to the current optical beamguiding system.The application of only 100 laser pulses resulted on both

materials in rather shallow depressions. In this initial stage ofthe LA process the Cu crater consists of a dense network ofelongated and curved ripples which are randomly distributed(see panel b of Figure 2). These morphological features showpronounced similarities to nanoprotrusions and nanocavitiesreported by Vorobyev et al. for Pt and other metallic surfaces.40

In contrast to Cu, we observe on Si larger droplet-like featuresin the initial stage of crater formation (Figure 2g).Differences in the lateral dimensions of zones affected by the

laser pulses are more apparent in the initial stage of craterdevelopment (100 laser shots) than in a more advanced stageof the LA process (10 000 laser shots). These differences in thedimensions of the affected zones can be rationalized on thebasis of the material specific thermal conductivity which issubstantially higher in case of Cu (see Table-S1).41 A higherthermal conductivity implies much faster and more effectivedissipation of heat into the bulk material, therefore resulting ina spatially and temporally more confined impact of repetitivelaser pulses on the sample surface.In particular, the wings of the quasi-flat-top laser pulse whose

energy is below the actual ablation threshold can induce severemorphological changes in the peripheral zones affected by thelaser pulse even without any net removal of sample material.

These heat-mediated morphological damages are morepronounced and laterally more extended for materials like Sihaving a lower thermal conductivity. These morphologicalchanges often go along with significant changes in the localoptical properties of the material. An increased surfaceroughness typically leads to a more effective absorption oflaser light, which further reduces the so-called “damagethreshold”.This kind of incubation phenomena was already reported in

the literature.42 They were found to alter the optical propertiesof the material even for laser pulses as short as 5 fs.42 Typicallythe time scale of the exciting laser pulse in these experiments isabout 3 orders of magnitude shorter than the one on whichvibrational excitations (lattice heating) take place. This is thereason why the heat affected zone (HAZ) is significantlyreduced when using sub-ps pulses. However, the initial electronheating leads to an electron−phonon coupling which canthermally alter the close vicinity (few nm) of the generated LAcrater.43

Figure 2d−e, i−j display HR-SEM images of those sectionsof the LA crater features (10 000 laser shots) highlighted bygreen and blue frames in panels c and h. The LA crater formedin the Si substrate exhibits a nm-smooth surface appearance. Inaddition, so-called laser-induced periodic surface structures(LIPSSs), such as ripples and columns, are visible on the μmlength scale.44 This surface pattern might originate from theinterference of the incident laser beam with a surfaceelectromagnetic wave (SEMW) of the target material thatinvolves the collective excitation of electron modes (e.g.,surface plasmon polaritons (SPPs)).45,46 A further difference inthe crater characteristics of Si and Cu concerns a bright rimsurrounding the LA craters on the Si sample. At highermagnification in the SEM inspection (Figure 2j), one can

Figure 3. Panels (a) and (b) show cross-sectional AFM depth profiles of Si and Cu craters recorded along the scanning line indicated in the insetimage of panel a). Panels (c) and (d) show the crater diameter measured at 2 μm crater depth (green dotted line in panels a and b) as a function ofpulse energy for 10 000 laser shots and as a function of applied laser shots for a pulse energy of ∼3.3 μJ, respectively. The blue dotted line only servesto guide the eye and should illustrate a nonlinear behavior of the data evolution.

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ascribe these structural features to resputtered Si nanoparticles(∼100 nm in size). Such resputtering effects were not observedfor the Cu sample. In contrast to the Si, the Cu craters exhibit arougher surface appearance on the nm scale. A certain kind ofextended, μm-sized LIPSS features are also present at theopening and the side-walls of the Cu crater (Figure 2d), but toa lesser extent than observed for Si. They are most likelyformed by the growth and subsequent coalescence of so-callednanotubes (Figure 2b) by increasing the number of appliedlaser pulses.40 An overview of all top-down SEM images of theSi and Cu craters is provided in the SI file (see Figures S-2−S-3)Further structural characterization of the crater openings was

conducted by means of AFM (Figure 3). Note that due to thelarge aspect ratio of the LA craters the AFM methodology isreliable only for crater depths down to ∼4 μm. All LA craterswere analyzed along the two main symmetry axes of theelliptical cavities (ratio of both axes is about 0.8). For the sakeof simplicity we show here only those AFM measurementsrecorded along the scanning direction hitting the center of thecraters as indicated in the inset figure of Figure 3a. Panels a andb in Figure 3 present topographic profiles of LA cratersgenerated after applying 100, 1000, 3000, and 10 000 lasershots at a pulse energy of 3.3 μJ. Note that the smaller depthfor the crater formed by 10 000 laser shots shown in Figure 3aemerges due to AFM measurement artifacts and is in factdeeper than the other presented craters. To avoid artifacts inthe AFM imaging related to the large aspect ratio, the presentedcrater profiles result from the superposition of forward andbackward AFM scans as illustrated by the inset in Figure 3b.The AFM results indicate LA crater profiles in close vicinity ofthe crater opening, which are only weakly affected by the totalnumber of applied laser shots (n). This observation supportsthe quasi-flat-top nature of the employed fs-laser beam. Theobserved secondary depressions around the major crater arebelow one micrometer in depth, and are believed to be anaccumulation of LIPSS features, which may emerge from thebeam wings.Figure 3c presents the dependence of the LA crater

diameters at a depth of 2 μm on the laser pulse energies. ForCu there is clearly a nonlinear increase of the crater diameter inthe range of lower pulse energies, which, however, reaches a

plateau regime at larger pulse energies. Because the craterdiameter is determined by the shape of the laser pulse profile,the origin of the plateau is most likely due to the steep walls ofour quasi-flat-top pulse profile, whereas the initial steep increaseof the curve is indicative for a slight deviation from the idealflat-top shape of the laser beam dominating the crater evolutioncharacteristics particularity at lower pulse energies. Note thatthe scatter of the respective AFM data for Si does not allow anyconclusive statement for the Si craters.The plot in panel d of Figure 3 illustrates the dependence of

the crater diameter on the total number of applied laser shots atconstant applied pulse energy of 3.3 μJ. Contrary to what isexpected from a Gaussian beam profile,28 only a moderatechange of the crater diameters with the applied number of lasershots is observed for Si and Cu craters. An increase of the craterdiameter for both materials is solely noticeable in the initialphase of the crater evolution process before entering a plateauregime at higher numbers of laser shots. These findings areindicative for LA mechanism which might change as the craterevolves.Both target materials form craters with similar diameters

upon increasing pulse energies and increasing number ofapplied laser shots, at least within the investigated ranges.

Advanced Crater Analysis. To go beyond a pure top-down crater inspection we applied two advanced preparationmethods to unravel the depth and the full three-dimensionalshape of the formed LA craters. The first one was based on alithographic approach followed by a selective and anisotropic,wet chemical etching process of Si leaving behind cross-sectioned LA craters. The second one applied to the Cu samplewas based on a PDMS process yielding a full three-dimensionalreplica as casting mold of the crater features (for details seesection Materials and Methods).Survey SEM images of the analyzed matrices of crater cross

sections and molds are presented in Figure 4a,c. RepresentativeSEM images of a closed-up single cross-sectioned Si crater anda single Cu crater replica are shown in Figure 4b,d, respectively.Both craters were produced by applying the same number of3000 laser shots at constant pulse energy of 1.4 μJ. Our SEManalysis reveals for both cases a distinct conical shape of the LAcraters, although the laser beam has been optimized for a quasi-flat-top beam profile prior to the LA treatment. LIPSS features

Figure 4. (a) and (c) SEM overview of Si crater cross sections obtained by the “Bosch-process” and PDMS mold of the Cu craters. Magnified SEMimages showing a typical (b) cross-section of a Si crater and (d) PDMS mold of a Cu crater (3000 shots at ∼1.4 μJ). Both approaches enabled thedetermination of the crater depth and profile for different applied measurement conditions (pulse energies and number of laser shots).

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already observed in the top-down SEM crater inspection(Figure 2) are also clearly visible at the interior side-walls of thecrater features (Figure 4b) and on the outer surface of thePDMS mold (Figure 4d), the latter representing themorphology of the side-walls inside the respective Cu crater.1

A comprehensive analysis of the obtained SEM data was usedas basis for a detailed quantification of the LA characteristics onSi and Cu. Results are presented in Figure 5 for a number oftotal laser shots ranging from 100 to 10 000 and pulse energiesthat were varied from 0.9 to 3.3 μJ. Note that due to a slightmisalignment of the polymer mask during the Si etchingprocess, craters formed at pulse energies of 3.0−3.3 μJ were notcross-sectioned properly during the process and were thereforenot included into our analysis. Data presented in panels a and bof Figure 5 indicate LA depths, which are 3 times lower for Si ascompared with Cu.Electrons are excited in the semiconducting Si from the

occupied valence band to the unoccupied conduction bandwith a photon energy that exceeds the bandgap, whereas inmetals the light absorption primarily proceeds via free electronsfrom the conduction band. Therefore, higher energy densitiesare required for Si to generate free electrons.47

From a thermodynamic point of view one can assume in afirst approximation a scaling of the ablation and ionization for aspecific material with (i) its melting/boiling point, (ii) itsatomization enthalpy, and (iii) its (first, second) ionizationenergy. The atomization enthalpy is the minumum energyneeded to break chemical bonds in the solid and to promote

the constituting atoms/molecule from the solid to the gaseousphase. From this simplified considerations (see Table S-1) onewould indeed expect a higher ablation rate for Cu than for Si.For both materials, the crater depth scales with the applied

pulse energy and shows an inverse exponential dependence onthe applied number of laser shots. This behavior is fullyconsistent with a LA rate that decreases as a function of thecrater depth and with the evolution of conical cratergeometries. For the very initial phase of crater evolution (firstfew laser shots), there is a distinct correlation between the LArate and the total applied energy (Figure 5c,d). For Cu, weobserve a constant LA rate for up to 500 laser shots.Interestingly, for the first 100 laser pulses the LA rate of Si isby a factor of about 2 higher as compared to the one of Cu. TheLA rate of Si drops down much faster upon increasing numberof applied laser shots than the one of Cu, resulting in lowercrater depths in the Si for an advanced stage of the craterevolution. Further information about the dependence of thetotal area affected by the laser irradiation (determined by top-down SEM analysis) on the total applied energy can be foundin the SI (see Figure S-4).A preliminary determination of the crater volumes (Figure 6)

was based on the obtained SEM and AFM data (crater depthsand crater diameters) under the simplifying assumption of anideally conical shape of the formed LA craters. A possibleexplanation for the resulting conical shape of the craters couldbe related to the nature of the applied telescope that is used forthe transformation of the initial Gaussian beam to the flat-top

Figure 5. Panels (a) and (b) show ablation depths as a function of the applied laser shots for different applied pulse energies. Panels (c) and (d)present the changes of the mean ablation rate, calculated over the crater depth, for different pulse energies, with increasing applied total energy(number of laser shots times the pulse energy).

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profile. The beam after the beam shaper undergoes a sequenceof interferometric patterns, which should create in the lens’focal plane a profile that is close to a flat-top profile.Displacement from the focal plane modifies the pulse profileup to the restoration of a Gaussian-like beam profile. Thiswould require a constant adjustment of the focusing lens, whichat this state is not applicable due to the instrument design.A more precise profile determination can be done by

extracting the PDMS profile from SEM images, as exemplarilyperformed for one prototypical crater mold presented in the SI(see Figure S-6). However, such high-accurate imageextractions and direct experimental determination of the cratervolumes (e.g., by means of confocal microscopy, white-lightinterferometry, or micro-CT) is beyond the scope of ourpresent paper and will therefore be addressed separately in aforthcoming communication.Panels a and b in Figure 6 show the calculated wall angle (i)

as a function of applied pulse energy for a constant number oflaser shots and (ii) as a function of the number of applied lasershots at a constant pulse energy. The calculated cone angles at2 μm crater depth (see red dotted line in Figure 6c for theillustration of the angle) are clearly larger for Cu than for Si.Moreover, upon applying a constant large number of laser shots

for different pulse energies the angle of the assumed cone craterseems to fluctuate much less for Cu than for Si (Figure 6a).However, for low numbers of laser shots, the cone angledepends exponentially on the amount of deposited energy andtherefore on the initial crater formation step (Figure 6b). Thisimplies that the ratio between the base and the depth of thecraters is almost constant for different applied pulse energiesonce the crater shape has been established.Figure 6d shows the increase of the crater volume with

increasing energy dose. It becomes obvious that, at a givenenergy dose, the LA craters on Cu are much bigger in volumethan the ones formed on the Si substrate thus pointing to LAprocesses which are much more effective on Cu than on Si (asexpected from the specific material characteristics). The opensquares in Figure 6d represent the evolution of the cratervolumes with increasing number of laser shots for constantpulse energies of ∼3.3 μJ, whereas the filled symbols depictcrater volumes as a function of the pulse energy at a constantnumber of applied laser shots (10 000). In both types ofexperiments, the resulting dependence of the ablated volumedecreases with increasing total energy dose.

Figure 6. Panels (a) and (b) present the calculated cone angle from the red dotted line in (c) derived from the measured radius and depth at 2 μmcrater depth as a function of the applied pulse energy or number of shots, respectively. Panel (c): Sketch of the cross-section of the crater illustratingthe difference between the real shape, indicated by the solid black line, and the simplified modeled cone shape shown as brown dotted line. The reddotted line shows the crater radius at 2 μm depth measured by AFM. Panel (d): Ablated crater volumes for Si and Cu craters assuming an ellipticalcone shape represented in (c) by the brown dotted triangle.

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■ CONCLUSIONSAnisotropic Si etching and PDMS casting techniques enable afast and reliable processing of two-dimensional arrays of LAcraters (about 80 craters per sample in this study) with the aimof a systematic both qualitative and quantitative analysis of thecrater evolution. These new preparation techniques allow for adirect correlation of the applied laser conditions (number oflaser shots and pulse energy) and the resulting cratermorphologies.Regardless of the sample material (Si or Cu) and the

particular preparation technique applied, our results clearlydemonstrate the formation of cone-shaped crater geometriesupon laser−matter interaction. Characteristic LIPSS featuresimaged on the crater openings by conventional top-down SEMinspection could now be observed even in the interior of theLA craters. Our novel LA crater preparation schemes provide abroad experimental platform for a deeper understanding oflaser-matter interaction in general and allow for thedetermination of various quantities that are important for theLA process such as the mean ablation rate and the cratervolumes. Important to note is that in particular the PDMScasting approach is nondestructive and therefore allows arepetitive formation of crater replica. Furthermore, thistechnique is much more versatile and applicable to a varietyof materials, whereas the etching process remains restricted tothose sample materials which can be selectively and anisotropi-cally etched by wet chemical processing (e.g., Si). Anotherdrawback of the DRIE technique is that the obtained cross-sectional information is restricted to one axis of the crater onlyand that this information relies on the precise etching along thecenter of the crater. For slightly asymmetric craters this etchingprocedure may result in incomplete depth profiles.The novel preparation techniques were applied to Si and Cu

model substrates. As expected from the material characteristics,our quantitative crater volume and crater depth analysisrevealed LA rates that are larger for Cu than for Si.Future work will particularly focus on the more accurate

determination of the LA crater volumes on the basis of PDMSreplica by using advanced three-dimensional optical microscopyand micro X-ray computer tomography.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.anal-chem.7b04539.

Surveys of optical microscope and top-down SEMimages of all Si and Cu craters, technical details of craterimaging measurements, an optical microscope image ofthe lithographical mask, correlation plots of laser affectedarea and total applied energy, microscale side-wall angledetermination, and a table with various Si and Cumaterial properties (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: valentine.grimaudo@dcb.unibe.ch. Phone: +41 31 63143 17.*E-mail: peter.broekmann@dcb.unibe.ch.

ORCIDValentine Grimaudo: 0000-0002-7010-5903

Andreas Riedo: 0000-0001-9007-5791Peter Broekmann: 0000-0002-6287-1042NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

We acknowledge the work from the technical staff of thedepartment of Chemistry and Biochemistry and the SpaceResearch and Planetary Sciences division at the University ofBern, Switzerland as well as the technical (U. Drechsler) andpermanent research staff (H. Wolf) at IBM Research − Zurich.Additionally, this study was performed with the support of theinterfaculty Microscopy Imaging Centre (MIC) of theUniversity of Bern, Switzerland. This work was founded bythe Swiss National Science Foundation (SNSF, grant numbers200020_172507; 200020_149224, P2BEP2_165378 andCR22I2_152944).

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